It must be used whilst hot, and as long as it retains even slight warmth can be readily moulded into any form. It is often employed in making shallow cells for liquids, as before mentioned. GUM-WATER is an article which nobody should ever be without; but labels, or indeed any substance, affixed to glass with common gum, are so liable to leave it spontaneously, especially when kept very dry, that I have lately added five or six drops of glycerine to an ounce of the gum solution. This addition has rendered it very trustworthy even on glass, and now I never use it without. This solution cannot be kept long without undergoing fermentation, to prevent which the addition of a small quantity of any essential oil (as oil of cloves, &c.), or one-fourth of its volume of alcohol, may be made, which will not interfere in any way with its use. There is what is sometimes termed an _extra adhesive_ gum-water, which is made with the addition of isinglass, thus:--Dissolve two drachms of isinglass in four ounces of distilled vinegar; add as much gum arabic as will give it the required consistency. This will keep very well, but is apt to become thinner, when a little more gum may be added. All these, except one or two, are liquid, and must be kept in _stoppered_ bottles, or, at least, as free from the action of the air as possible. When any two substances are to be united firmly, I have termed the medium employed “a cement;” but often the appearance of the slides is thought to be improved by drawing a coloured ring upon them, extending partly on the cover and partly on the slide, hiding the junction of the two. The medium used in these cases I term A VARNISH, and hereinafter mention one or two. Of course, the tenacity is not required to be so perfect as in the _cements_. SEALING-WAX VARNISH is prepared by coarsely powdering sealing wax, and adding spirits of wine; it is then digested at a gentle heat to the required thickness. This is very frequently used to “finish” the slides, as before mentioned, and can easily be made of any colour by employing different kinds of sealing-wax; but is very liable to “chip” and leave the glass. BLACK VARNISH is readily prepared by adding a small quantity of lamp-black to gold-size and mixing intimately. Dr. Carpenter recommends this as a good finishing varnish, drying quickly and being free from that brittleness which renders some of the others almost worthless; but it should not be used in the first process when mounting objects in fluid. Amongst these different cements and varnishes I worked a long time without coming to any decision as to their comparative qualities, though making innumerable experiments. The harder kinds were continually cracking, and the softer possessed but little adhesive power. To find hardness and adhesiveness united was my object, and the following possesses these qualities in a great degree:-- India-rubber ½ drachm. Asphaltum 4 oz. Mineral naphtha 10 ”